This application claims the benefit of Korean Patent Application No. 2000-63685 filed on Oct. 28, 2000, which is hereby incorporated by reference as if fully set forth herein.
1. Field of the Invention
The present invention relates to photoalignment materials and to their use in liquid crystal displays.
2. Discussion of the Related Art
There is significant interest in liquid crystal displays because of their lightweight and low power consumption. A liquid crystal display includes a liquid crystal panel that is formed by aligning two prepared substrates at a constant interval and then injecting a liquid crystal between them. To produce a high quality image a liquid crystal display requires good light transmission, fast response time, wide viewing angle, and good contrast.
Because these requirements depend on the alignment characteristics of the liquid crystal molecules used in the display, good alignment characteristics are important. Unfortunately, good alignment is not accomplished by simply injecting a liquid crystal between substrates. In practice an alignment film is required on at least one of the substrates. Such an alignment film should have an anisotropic structure similar to that of the liquid crystal molecules.
Polyimide resin alignment films are typically used because they have good heat resistance and an affinity for various liquid crystals. A polyimide alignment film is usually prepared by printing polyamic acid on a substrate, drying that acid, and performing an imidization process. Then, the polyimide alignment film is rubbed to induce the desired anisotropic property.
The rubbing process generally involves inducing a directional alignment by directly rubbing the alignment film with a specifically manufactured cloth. This rubbing process is widely used for manufacturing liquid crystal displays because rubbing is simple and easy to perform.
Unfortunately, rubbing is not appropriate for inducing regular alignment over a large area as various rubbing parameters are difficult to control. Furthermore, a rubbed alignment film can be contaminated by fine particles and fibers produced by rubbing. Additionally, thin film transistors that are used in liquid crystal displays as switching element can be damaged by static electricity generated by rubbing. These problems can result in a defective or low quality liquid crystal display.
Because of the problems with rubbing, non-contact alignment methods are being developed. Those new methods include the Langmuir-Blodgett method, the fine image formation method, the MOLCA (Magnetically Oriented Liquid Crystalline Adsorption) method, and the photoalignment method.
The photoalignment method in particular has drawn significant attention because it potentially can result in a low cost, simple way of producing polymer alignment surfaces in a way that is suitable for mass production. Photoalignment uses light (typically ultraviolet or polarized ultraviolet) to produce an alignment layer. Additionally, Furthermore, the photoalignment method might also be able to improve viewing angle by forming multi-domain.
Photoalignment produced alignment films should be made of materials that are sensitive to a particular set of wavelengths that are appropriate for chemical and physical reactions. Reported photoalignment materials include polyvinylcinnamate and polyazobenzene. Those materials are known to form photosenstive polymers that can induce liquid crystal alignment.
However, prior art photoalignment materials have problems with establishing a desired pretilt angle and with thermal stability. Because prior art photoalignment materials can produce only very low pretilt it is difficult or impossible to get a desired pretilt angle. Furthermore, thermal stability is a problem because an alignment film""s stability is severely dependent on the thermal stability of the polymers per se. Prior art photoalignment materials having glass transition temperatures below 100xc2x0 C. are simply not appropriate.
Although attempts have been made to address the foregoing problems of prior art photoalignment materials, successful photoalignment materials were not achieved in the prior art.
Therefore, an object of the present invention is to provide a new photoalignment material.
Another object of the present invention is to provide a photoalignment material comprised of a diamine derivative with side branches, and which shows marked absorption of ultraviolet rays having 250-400 nm wavelengths.
Another object of the present invention is to provide a photoalignment material that has good photoalignment properties by using a diamine derivative as a main chain, and that increases pretilt angle by having side branches.
Still another object of the present invention is to provide a liquid crystal display device that can provide improved viewing angles by using an alignment film formed by a photoalignment material.
Additional features and advantages of the invention will be set forth in the description which follows, and in part will be apparent from the description, or may be learned by practice of the invention. The objectives and other advantages of the invention will be realized and attained by the structure particularly pointed out in the written description and claims hereof as well as the appended drawings.